1. Field of the Invention
The present invention relates to gyroscopes, and in particular to planar resonator gyroscopes or inertial sensors and their operation. More particularly, this invention relates to a planar resonator sensor used in an inertial measurement unit to sense motion for multiple degrees of freedom.
2. Description of the Related Art
An inertial measurement units (IMU) is a critical sensing system, particularly for aerospace applications such as aircraft or spacecraft vehicles. However, as the costs of IMUs are reduced, they may be employed in automobiles or any moving object. They provide comprehensive sensing of the relative motion of the vehicle without using a reference external to the vehicle. Typically, the IMU delivers acceleration sensing along three orthogonal axes as well as rotation rate sensing about three orthogonal axes to provide a complete representation of the vehicle movement. Position information may be readily derived from this sensed data, particularly when combined with position reference information. Conventional IMUs rely on multiple different physical sensors to provide the complete motion sensing. Typically, each individual sensor for the IMU is capable of sensing either along a single axis for acceleration or about a single axis for rotation. Thus, an IMU may utilize sensing information from three accelerometers each aligned to different orthogonal axes along with three gyroscopes each sensing rotation about three orthogonal axes. Providing rate sensing (e.g., from a gyro) is generally more challenging than providing acceleration sensing. Much work has been done in recent years to develop higher precision sensing (particularly gyroscopic sensing) at lower costs.
Conventional mechanical gyroscopes are used to determine direction of a moving platform based upon the sensed inertial reaction of an internally moving proof mass. A typical electromechanical gyroscope comprises a suspended proof mass, gyroscope case, pickoffs, torquers and readout electronics. The inertial proof mass is internally suspended from the gyroscope case that is rigidly mounted to the platform and communicates the inertial motion of the platform while otherwise isolating the proof mass from external disturbances. The pickoffs to sense the internal motion of the proof mass, the torquers to maintain or adjust this motion and the readout electronics that must be in close proximity to the proof mass are mounted to the case which also provides the electrical feedthrough connections to the platform electronics and power supply. The case also provides a standard mechanical interface to attach and align the gyroscope with the vehicle platform. In various forms gyroscopes are often employed as a critical sensor for vehicles such as aircraft and spacecraft. They are generally useful for navigation or whenever it is necessary to autonomously determine the orientation of a free object.
Older conventional mechanical gyroscopes were very heavy mechanisms by current standards, employing relatively large spinning masses. A number of recent technologies have brought new forms of gyroscopes, including optical gyroscopes such as laser gyroscopes and fiberoptic gyroscopes as well as mechanical vibratory gyroscopes.
Spacecraft generally depend on inertial rate sensing equipment to supplement attitude control. Currently this is often performed with expensive conventional spinning mass gyros (e.g., a Kearfott inertial reference unit) or conventionally-machined vibratory gyroscopes (e.g. a Litton hemispherical resonator gyroscope inertial reference unit). However, both of these are very expensive, large and heavy.
In addition, although some prior symmetric vibratory gyroscopes have been produced, their vibratory momentum is transferred through the case directly to the vehicle platform. This transfer or coupling admits external disturbances and energy loss indistinguishable from inertial rate input and hence leads to sensing errors and drift. One example of such a vibratory gyroscope may be found in U.S. Pat. No. 5,894,090 to Tang et al. which describes a symmetric cloverleaf vibratory gyroscope design and is hereby incorporated by reference herein. Other planar tuning fork gyroscopes may achieve a degree of isolation of the vibration from the baseplate, however these gyroscopes lack the vibrational symmetry desirable for tuned operation.
In addition, shell mode gyroscopes, such as the hemispherical resonator gyroscope and the vibrating thin ring gyroscope, are known to have some desirable isolation and vibrational symmetry attributes. However, these designs are not suitable for or have significant limitations with thin planar silicon microfabrication. The hemispherical resonator employs the extensive cylindrical sides of the hemisphere for sensitive electrostatic sensors and effective actuators. However its high aspect ratio and three-dimensional curved geometry is unsuitable for inexpensive thin planar silicon microfabrication. The thin ring gyroscope (e.g., U.S. Pat. No. 6,282,958, which is incorporated by reference herein) while suitable for planar silicon microfabrication, lacks electrostatic sensors and actuators that take advantage of the extensive planar area of the device. Moreover, the case for this gyroscope is not of the same material as the resonator proof mass so that the alignment of the pickoffs and torquers relative to the resonator proof mass change with temperature, resulting in gyroscope drift.
Most recently, some planar resonator gyroscopes devices have been developed (such as a disc resonator gyroscope) which operate through the excitation and sensing of in-plane vibrational modes of a substantially solid planar resonator. These planar resonators obtain enhanced properties over designs such as the hemispherical or shell resonators by enabling greater drive and sensing area in a compact package that is more easily manufactured and packaged. For example, see U.S. Pat. No. 6,944,931 by Shcheglov et al., issued Sep. 20, 2005 and entitled “INTEGRAL RESONATOR GYROSCOPE” and U.S. Pat. No. 7,043,163 by Shcheglov et al., issued May 9, 2006 and entitled “ISOLATED PLANAR GYROSCOPE WITH INTERNAL RADIAL SENSING AND ACTUATION.”
Further limitations arise in the current art of multiaxis inertial sensors or IMUs that operate with reduced performance or at a significantly increased size or cost beyond that of a single-axis gyroscope. Multiple single-axis vibratory gyroscopes of the same manufacture have similar but not identical resonant frequencies. Accordingly, separate control frequencies requiring separate drive electronics and control loops are used for each and interfere with one another electrically or mechanically when mounted in the same vacuum housing. The alternative of mounting them in separate isolated vacuum housings with separate readout electronics is expensive since vacuum packaging is typically more than half the cost of a sensor. In addition, the temperature and vacuum levels for each sensor may vary independently resulting in the need for separate compensation electronics.
In view of the foregoing, there is a need in the art for compact, inexpensive, inertial measurement units. There is a need for such inertial measurement units for navigation and spacecraft payload pointing and other applications. To this end, there is also a need for systems and methods to operate planar resonators to yield sensing motion for multiple degrees of freedom including acceleration sensing along three orthogonal axes and rotation sensing about three orthogonal axes. There is need to have a single vacuum package for all three gyroscopes to reduce cost and have the same vacuum level, temperature and drive frequency for all three gyroscopes and preferably the same driven motion used for sensing all three axis rates. As detailed below, the present invention satisfies these and other needs.